1. Field of the Invention
This invention relates to a test solution and, more particularly to a solution which can be used to check photometric accuracy and linearity.
2. Description of the Prior Art
Frings et al. (1) report that the percentage of quantitative analyses performed in the clinical laboratory that involve spectrophotometry or colorimetry was estimated in 1969 to be possibly more than 95% (2). Most laboratories continue to rely heavily upon spectrometer or spectrophotometers for the majority of their analyses. Maintenance of properly functioning spectrometers and spectrophotometers is an obvious prerequisite to the assurance of accurate analytical results. Moreover, the increased regulation of clinical laboratory by governmental and professional agencies mandates that laboratory personnel periodically verify that a given spectrometer or spectrophotometer is functioning properly. By periodically inspecting spectrometric and spectrophotometric functions, subtle or gradual degradations in performance can be detected before they significantly affect analytical results. As a minimum, these inspections should include, inter alia, checks for photometric accuracy and linearity of detector response.
With respect to linearity of detector response, a properly functioning spectrometer or spectrophotometer must exhibit a linear relationship between the radiant energy absorbed and the instrument readout (3). Instrument linearity is a prerequisite for spectrometric and spectrophotometric accuracy as well as for analytical accuracy. Several methods have been proposed for certifying that the detector response of spectrometers and spectrophotometers is linear over the range of wavelengths used (2-5).
The most common method for certifying linearity of detector response is through the use of solutions of compounds known to follow Beer's Law. Although it has been stated that the fulfillment of Beer's Law is a necessary but not sufficient condition for spectrometer and spectrophotometer linearity (4), the alternative procedures suggested for checking linearity are so elaborate and tedious that the use of solutions remain the most practical procedure available in the clinical laboratory.
The Subcommittee on Spectrophotometry of the Standards Committee of the American Association for Clinical Chemistry recommended in 1969 that photometric linearity be checked by preparing dilutions of appropriate compounds (2). Compounds reported for this purpose (2, 5) include oxyhemoglobin at 415 nm, p-nitrophenol at 405 nm, cobalt ammonium sulfate at 512 nm, copper sulfate at 650 nm and cyanmethemoglobin at 540 nm.
With respect to photometric accuracy, when performing analyses that do not use chemical standards, absorbance accuracy is essential. An absorbance standard (3) should have constant, stable absorbance over a suitable wavelength range that is insensitive to the spectral bandwidth of the instrument and to variations in the geometry of the light beam; and it should be easy to use, readily available, and inexpensive.
In general, linearity of detector response involves measuring absorbance (A) or percent transmittance (%T) of solutions having four different concentrations of cobalt ammonium sulfate present therein at a given wavelength. The concentration of cobalt ammonium sulfate present in each solution differs in an arithmetic progression advancing from 0 concentration of cobalt ammonium sulfate to a desired upper limit present in the fourth solution.
In general, the photometric accuracy technique involves measuring A or %T of the photometric accuracy test solution at a given wavelength.
A solution of cobalt ammonium sulfate and sulfuric acid (0.37 normal H.sub.2 SO.sub.4) has been employed to check linearity of the detector response as well as photometric accuracy. The amount of sulfuric acid present in these solutions is relatively caustic, and, therefore, requires special caution with regard to shipping and handling and, in addition, causes excessive wear on those parts of the instrument with which it comes in contact (Rand, Clin. Chem., 15 pp. 839-863(1969)).
Accordingly, it would be desirable to use a cobalt ammonium sulfate solution in a form devoid of these attendant disadvantages.